Technology update

Oct 7, 2008

Nanomatrix puts tungsten within reach of goal

Placing tungsten trioxide within a nanoporous matrix such as carbon inverse opal, carbon nanotube paper or platinum sponge and then intercalating the structure with alkaline ions may turn out to be recipe for superconducting success. Ali Aliev and his colleagues from the University of Texas at Dallas, US, have observed magnetic and electrical anomalies in various samples of the material (LixWO3–y and NaxWO3–y) over a temperature range of 125–132 K.

Aliev points out that the team's published results provide only partial evidence of superconductivity, but the group is working hard to establish the full story and has high hopes for its material.

"The extremely high surface area of tungsten bronzes infiltrated into shell-type inverted carbon opals with very tiny heat capacity and low thermal conductivity could be employed in various sensors like IR bolometers, SQUIDs and nano switch devices for microwave detectors in MRI," Aliev told nanotechweb.org.

The researchers deposit the tungsten oxide film on to the host porous matrix by dipping a bar-shaped test structure into peroxotungstic acid. Next, they apply a vacuum to encourage the solution to penetrate deep into the nanoporous network.

To get a higher filling factor, the dipping process is repeated at least 4–5 times. After each infiltration, the test material is left to dry at room temperature for 10 minutes and then sintered at 130 °C for 30 minutes to help fix the freshly deposited film on to the host surface.

Electrochemical intercalation is performed by connecting the tungsten rich matrix to a three electrode set-up featuring various electrolytes.

Upon investigation, the treated material displays magnetic and electrical behaviour that suggests the possibility of localized non-percolated superconductivity, although why nanostructured matrices are able to exert this effect is up for debate.

"The behavior of electronic states under light excitation in tungsten bronzes suggests a bipolaronic mechanism of electron coupling. Regarding the nanostructure, perhaps the quantum confinement of carriers within the small islands (10–15 nm) leads to discretization of the quasiparticle density-of-state, which can account for the slight increase in the transition temperature from 91 K for bulk tungsten bronze to 125 K for nanostructured ones," commented Aliev. "However, I believe that below ~5 nm the superconductivity is totally suppressed due to the lack of any states around the bandgap."